This invention relates to an ignition apparatus for a spark-ignited internal combustion engine. More particularly, it relates to an ignition apparatus which controls ignition timing of a spark plug by means of a signal which is generated by a signal generator in synchrony with the rotation of an engine.
In order to carry out proper ignition in an internal combustion engine, it is necessary for the current flowing through the primary winding of the ignition coil for the engine to reach a prescribed level. On the other hand, if the primary winding current exceeds this prescribed level, electric power from the storage battery for the engine, which is used to energize the ignition coil will be needlessly consumed.
Therefore, many engines are equipped with ignition controllers which prevent the primary winding current from exceeding a prescribed current limit. A typical example of an ignition apparatus including such an ignition controller is illustrated in FIG. 4. The ignition apparatus illustrated includes a signal generator 1 which generates an alternating output signal in synchrony with the rotation of an internal combustion engine. A resistor 2 is connected at its one end with the positive terminal of a DC power source 50 and grounded at its other end through a resistor 3 and a pair of serially connected diodes 4, 5. A pair of transistors 6, 7 have their collectors commonly connected with the positive terminal of the DC power source 50, and their emitters commonly connected with one end of the signal generator 1 through a first external connection point C1 and at the same time commonly grounded through a resistor B. A transistor 9 has a base coupled with a connection point between the resistors 2, 3, a collector coupled with the positive terminal of the DC power source 50, and an emitter grounded through a resistor 10. A wave form shaper 12 has a first positive input terminal connected with the signal generator 1 through a second external connection point C2, a second negative input terminal coupled with the emitter of the transistor 9 through a resistor 11, and an output terminal coupled with an input side of a driver circuit 16 for driving a power transistor circuit 17. A transistor 14 has a base coupled via a resistor 15 with the output terminal of the wave form shaper 12, a collector coupled via a resistor 13 with the second negative input terminal of the wave form shaper 12, and an emitter grounded. The driver circuit 16 is connected at its output side with the power transistor circuit 17 which includes a pair of transistors 17a, 17b. The transistor 17a has a base coupled with the output side of the driver circuit 16, a collector coupled with a collector of the transistor 17b, and an emitter coupled with a base of the transistor 17b which has an emitter grounded through a resistor 19. The ignition coil 18 includes a primary winding connected at its one end with the positive terminal of the DC power source 50 and at its other end with the collectors of the transistors 17a, 17b, and a secondary winding which is connected with a spark plug (not shown). A current detecting circuit 21 has a positive input terminal coupled with the emitter of the transistor 17b, a negative input terminal coupled with the positive terminal of a DC power source 20, and an output terminal coupled through a resistor 22 with a base of a transistor 23. The transistor 23 has an emitter grounded and a collector coupled with one end of a resistor 24 which is coupled at the other end thereof with the other end of the signal generator 1 through the second external connection point C2. The resistor 24 is coupled at one end with an anode of a diode 25 which is in turn coupled at its cathode with one end of a capacitor 26 which is grounded at its other end. A buffer circuit 28 has a positive input terminal coupled with a connection point between the cathode of the diode 25 and the capacitor 26, a negative input terminal coupled with an output terminal thereof, and an output terminal with a base of the transistor 7.
All the above-described elements other than the signal generator 1, the ignition coil 18 and the DC power source 50 constitute an ignition controller which is electrically connected with the signal generator 1 through the first and second external connection points C1 and C2.
The above-described ignition apparatus operates as follows. The signal generator 1 generates, in synchrony with the rotation of the engine, an alternating ouput signal which is inputted to the wave form shaper 12 where the alternating output signal is shaped into an appropriate pulse signal. Namely, the wave form shaper 12 compares the input signal inputted to the first positive input terminal from the signal generator 1 with a reference or threshold voltage which is imposed upon the second negative input terminal of the wave form shaper 12 by the DC power source 50 through the transistor 9 and the resistor 11. When the alternating output signal of the signal generator 1 fed to the positive input terminal of the wave form shaper 12 is greater than the reference or threshold voltage at the negative input terminal thereof, the wave form shaper 12 generates an output signal in the form of a square pulse. The pulse signal thus generated is inputted through the driver circuit 16 to the power transistor circuit 17 for turning it on and off. Specifically, when an output signal of the driver circuit 16 is imposed upon the base of the transistor 17a, the transistor 17a is made conductive whereby a current is supplied through the now conductive transistor 17a to the base of the transistor 17b. As a result, the transistor 17b is also made conductive so that a current flows from the positive terminal of the DC power source 50 to ground through the primary winding of the ignition coil 18, the transistor 17b and the resistor 19.
On the other hand, when the driver circuit 16 stops generating the output signal, the transistors 17a, 17b are made non-conductive so that the current flowing through the primary winding of the ignition coil 18 is interrupted. Upon interruption of the primary winding current, a high voltage is developed in the secondary winding whereby the unillustrated spark plug electrically sparks, thus igniting a cylinder of the engine. The output signal of the wave form shaper 12 is also fed to the base of the transistor 14 through the resistor 15 whereby the transistor 14 is made conductive, thus changing the reference or threshold voltage level at the second negative input terminal of the wave form shaper 12.
In the above ignition apparatus, the duty cycle of the power transistor circuit 17 which controls the primary winding current of the ignition coil 18 is controlled in accordance with the voltage across the capacitor 26. The capacitor 26 is charged by the output signal of the signal generator 1 through the resistor 24 and the diode 25. When the engine is operating at a speed (i.e., in a high speed range) such that the charged voltage across the capacitor 26, which increases in accordance with increasing rotational speed of the engine and which is inputted to the first positive terminal of the buffer circuit 28, exceeds a prescribed level corresponding to a reference voltage at the negative input terminal of the buffer circuit 28, the buffer circuit 28 produces an output signal which is inputted to the base of the transistor 7 so that the transistor 7 is made conductive. The output signal of the buffer circuit 28 is also fed back to the negative input terminal thereof so as to provide the reference voltage. With the conduction of the transistor 7, a direct current is fed from the DC power source 50 to the signal generator 1 through the transistor 7 and the first external connection point C1 so as to raise the entire level of the alternating output signal of the signal generator 1 fed to the wave form shaper 12. As a result, the wave form shaper 12 generates an output signal in the form of a square pulse having a pulse width sufficient for providing an adequate primary winding current for the high speed operation of the engine.
When the primary winding current reaches an upper limit, the signal inputted to the first positive input terminal of the current detecting circuit 21 becomes equal to the reference or threshold voltage at the second negative input terminal so that the current detecting circuit 21 generates an output signal which is fed to the base of the transistor 23 through the resistor 22 whereby the transistor 23 is made conductive. As a result, the output signal of the signal generator 1 flows through the resistor 24 and the transistor 23 to ground while bypassing the capacitor 26. Accordingly, the transistor 7 is made nonconductive, interrupting the current supply from the DC power source 50 to the signal generator 1. In this manner, the duty cycle of the power transistor circuit 17 is controlled so that the interrupting current of the ignition coil 18 is made constant even in the high speed operating range of the engine, thus preventing a wasteful consumption of electric power.
When the engine is rotating at a low speed and the voltage across the capacitor 26 is below the prescribed level, the buffer circuit 28 generates no output signal so that the transistor 7 is non-conductive, interrupting the current supply from the DC power source 50 to the signal generator 1.
The above-described ignition apparatus, however, possesses the following disadvantages: 1) the circuit arrangement for controlling the duty cycle is relatively complicated; 2) the number of component parts is relatively large; 3) there are two external electrical connection points formed between the signal generator and the ignition controller; and 4) due to the above problems, the reliability of the entire apparatus is poor, and the manufacturing cost becomes high.